So What are you going to do when somebody calls you as an emergency?
Drop all of your work booked in? or tell them you will fit them in when you can ?
I would suggest emergency requires you immediately, I do get calls where they want you there as soon as possible and I tell them I will get there as soon as I can, my charge would be no different to my normal charge as it can lead to more work, unless of course its a call in the middle of the night.
Geoff